How to make a 3D Matrix

How to make a 3D Matrix that contains all ones in the first layer, all twos in the second layer and all threes in the third layer?

[Fangjun Jiang]

define layer

[David Hill]

Do you mean something like:

f(1:5,1:5,1)=1;
f(1:5,1:5,2)=2;
f(1:5,1:5,3)=3;

[Nicholas Deosaran]

Yes, but is there a way to do it with the ones function or no?

[Abdolkarim Mohammadi]

Size = [5,6,3];
M = ones(Size) .* permute([1;2;3],[3,2,1]);

[Nicholas Deosaran]

Thank you!

Answer 1

[Jon]

You can also do it like this, for example for a 2 by 4 by 3

A = ones(2,4,3).*reshape(1:3,1,1,3)

[Nicholas Deosaran]

Jon thank you for that as well!

[Nicholas Deosaran]

what does the reshape(1:3,1,1,3) fucntion do?

[Abdolkarim Mohammadi]

A simple way to reach the matrix you wanted is to create a m-by-n-by-k array of ones and then multiply it by a 1-by-1-by-k array of 1 to k. The trick is to first create a 1-by-k array and then permute it to ge the so-called 1-by-1-by-k array. You can permute it in different ways, including the permute() function and reshape().

[Jon]

You asked about what the reshape function does.

The reshape(1:3,1,1,3) The first argument 1:3 makes a 3 element row vector with elements [1, 2, 3]the second, third and fourth arguments specify that we want to turn this row vector into a 1 row, by 1 column, by 3 "page" array. So 1 goes in the 1,1,1 location of the first page, 2 goes into th 1,1,2 location on the second page and 3 goes into the 1,1,3 location on the third page. By shaping it this way the array dimension for the multiplication

ones(2,4,3).*reshape(1:3,1,1,3)

become "compatible". Please see https://www.mathworks.com/help/matlab/matlab_prog/compatible-array-sizes-for-basic-operations.html#:~:text=In%20the%20simplest%20cases%2C%20two,element%2Dwise%20operation%20or%20function.

Also type doc reshape on the command line to get documentation on the reshape function for further details